Repeating Append operation

<div dir="ltr">Hi all,<br /><br /><span style="font-family: courier new,monospace;">explain </span><br
style="font-family:courier new,monospace;" /><span style="font-family: courier new,monospace;">select v from
(</span><brstyle="font-family: courier new,monospace;" /><span style="font-family: courier new,monospace;">select
array(</span><brstyle="font-family: courier new,monospace;" /><span style="font-family: courier new,monospace;">       
select1</span><br style="font-family: courier new,monospace;" /><span style="font-family: courier
new,monospace;">       union all</span><br style="font-family: courier new,monospace;" /><span style="font-family:
couriernew,monospace;">        select 2) as v</span><br style="font-family: courier new,monospace;" /><span
style="font-family:courier new,monospace;">from (select 1) ) as s</span><br style="font-family: courier new,monospace;"
/><spanstyle="font-family: courier new,monospace;">where v is not null;</span><br style="font-family: courier
new,monospace;"/><br style="font-family: courier new,monospace;" />The plan looks like:<br /><br /><span
style="font-family:courier new,monospace;">                       QUERY PLAN</span><br style="font-family: courier
new,monospace;"/><span style="font-family: courier
new,monospace;">--------------------------------------------------------</span><brstyle="font-family: courier
new,monospace;"/><span style="font-family: courier new,monospace;"> Result  (cost=0.08..0.10 rows=1 width=0)</span><br
style="font-family:courier new,monospace;" /><span style="font-family: courier new,monospace;">   One-Time Filter: ($1
ISNOT NULL)</span><br style="font-family: courier new,monospace;" /><span style="font-family: courier
new,monospace;">  InitPlan</span><br style="font-family: courier new,monospace;" /><span style="font-family: courier
new,monospace;">    ->  Append  (cost=0.00..0.04 rows=2 width=0)</span><br style="font-family: courier
new,monospace;"/><span style="font-family: courier new,monospace;">           ->  Result  (cost=0.00..0.01 rows=1
width=0)</span><brstyle="font-family: courier new,monospace;" /><span style="font-family: courier
new,monospace;">          ->  Result  (cost=0.00..0.01 rows=1 width=0)</span><br style="font-family: courier
new,monospace;"/><span style="font-family: courier new,monospace;">     ->  Append  (cost=0.00..0.04 rows=2
width=0)</span><brstyle="font-family: courier new,monospace;" /><span style="font-family: courier
new,monospace;">          ->  Result  (cost=0.00..0.01 rows=1 width=0)</span><br style="font-family: courier
new,monospace;"/><span style="font-family: courier new,monospace;">           ->  Result  (cost=0.00..0.01 rows=1
width=0)</span><brstyle="font-family: courier new,monospace;" /><span style="font-family: courier new,monospace;">  
-> Result  (cost=0.00..0.01 rows=1 width=0)</span><br style="font-family: courier new,monospace;" /><span
style="font-family:courier new,monospace;">(10 rows)</span><br style="font-family: courier new,monospace;" /><br />   
Itseems that that the UNION ALL part of the query will be executed twice. If I remove the WHERE clause the I see only
oneAppend operation. I had a suspicion that its just the display of the plan that showed the same plan twice, but then
Inoticed that the overall cost of the query also drops making me think that this UNION ALL will actually be executed
twice.<br/><br />    The plan without the WHERE clause is:<br /><span style="font-family: courier
new,monospace;">                             QUERY PLAN</span><br style="font-family: courier new,monospace;" /><span
style="font-family:courier
new,monospace;">----------------------------------------------------------------------</span><brstyle="font-family:
couriernew,monospace;" /><span style="font-family: courier new,monospace;"> Subquery Scan __unnamed_subquery_0 
(cost=0.04..0.06rows=1 width=0)</span><br style="font-family: courier new,monospace;" /><span style="font-family:
couriernew,monospace;">   InitPlan</span><br style="font-family: courier new,monospace;" /><span style="font-family:
couriernew,monospace;">     ->  Append  (cost=0.00..0.04 rows=2 width=0)</span><br style="font-family: courier
new,monospace;"/><span style="font-family: courier new,monospace;">           ->  Result  (cost=0.00..0.01 rows=1
width=0)</span><brstyle="font-family: courier new,monospace;" /><span style="font-family: courier
new,monospace;">          ->  Result  (cost=0.00..0.01 rows=1 width=0)</span><br style="font-family: courier
new,monospace;"/><span style="font-family: courier new,monospace;">   ->  Result  (cost=0.00..0.01 rows=1
width=0)</span><brstyle="font-family: courier new,monospace;" /><span style="font-family: courier new,monospace;">(6
rows)</span><br/><br />    I had seen this with a bigger query on actual tables, and this is just a reproducible test
case.In the original query I see 'SubPlan' instead of the 'InitPlan' seen here.<br /><br />Head of plan with WHERE 
clause:<br/><span style="font-family: courier new,monospace;">  Seq Scan on "Person" p  (cost=0.00..280486580881.10
rows=1373415width=4)</span><br /><br />Head of plan without WHERE  clause:<br /><span style="font-family: courier
new,monospace;"> Seq Scan on "Person" p  (cost=0.00..140594841889.03 rows=1380317 width=4)</span><br /><br clear="all"
/>   Is there a way to avoid this double evaluation?<br /><br />Thanks in advance.<br />-- <br />gurjeet.singh<br />@
EnterpriseDB- The Enterprise Postgres Company<br /><a
href="http://www.enterprisedb.com">http://www.enterprisedb.com</a><br/><br />singh.gurjeet@{ gmail | yahoo }.com<br
/>Twitter/Skype:singh_gurjeet<br /><br />Mail sent from my BlackLaptop device<br /></div>